In designing the output stage of an apparatus which provides a direct current (DC) output signal, it is necessary to protect the output stage operational amplifiers from voltage surges inadvertently occurring on the output line. The operational amplifier may, for example, drive a twisted pair cable, a telephone line, or a co-axial conductor, etc.
Many output protection schemes have been devised for devices driving two wire lines. Such devices include modems which drive telephone lines. The output stages of the modem must be protected against ringing voltages on the telephone lines. Also, output stages which drive twisted pair cables must be protected against currents produced by wrong connections at the far end of the cable. The far end of the cable may inadvertently be connected to a source of voltage. Also, surges may be induced electromagnetically in the cables, etc.
A problem remaining in the field of protection of the output stage of operational amplifiers from incoming voltage surges on a cable being driven by the amplifier includes providing protection in a simple and inexpensive manner. The simple and inexpensive solution must also provide sufficient protection for the output stage operational amplifiers.